(1) Field of the invention
The present invention generally relates to an optical scanner Unit having a recursive optical system, and more particularly to an optical scanner unit having a recursive optical system which can be applied to an image reading apparatus.
(2) Description of related art
A first example of a conventional optical scanner unit is shown in FIG. 1. Referring to FIG. 1, a document 2 is put on a reading surface 1a of a main body 1 of this optical scanner unit. A light source 3 mounted in the main body 1 irradiates the surface of the document 2. A light reflected by the surface of the document 2 passes through an optical path extending to a CCD sensor 6. Mirrors 4 and a focusing lens 5 are arranged in the optical path, so that the light successively reflected by the mirrors 4 is focused by the focusing lens 5 on the CCD sensor 6. The light source 3, the mirrors 4 and focusing lens 5 are moved in a predetermined direction parallel to the reading surface 1a of the main body 1 so that the document 2 is optically scanned.
A second example of the conventional optical scanner unit is shown in FIG. 2. Referring to FIG. 2, a focusing lens 9 and a CCD sensor 10 are mounted in a region separated from a recording surface 8 on which a document 7 is put. The document 7 is irradiated by the light source 3 and optically scanned by moving a mirror (not shown in FIG. 2).
A third example of the conventional optical scanner unit is shown in FIG. 3. In the third example, a laser beam scans a document. Referring to FIG. 3, a laser beam emitted from a laser unit 11 is shaped into a predetermined form by a beam expander 12. The laser beam passing through the beam expander 12 is reflected by a polygonal mirror 13 which is rotated at a constant speed, so that the reflected laser beam is swung within a predetermined angle range. The swung laser beam is projected onto a surface of a document 15 via an f-.theta. lens 14, so that the laser beam scans the document 15 at a constant speed in a predetermined direction. While the laser beam is scanning the document 15, a light scattered on the document 15 is incident on an optical sensor 16. Image information is obtained based on output signals from the optical sensor 16.
In the third example of the conventional optical scanner unit, as a size of the laser beam spot can be small, the image formed on the document can be accurately read.
However, in an optical scanner unit using the CCD sensor, such as those of the first example shown in FIG. 1 and the second example shown in FIG. 2, as the light is strongly focused on the CCD sensor, a numerical aperture (N.A) in the optical system is large. Thus, a focal depth in the optical scanner unit is shallow. When a document having a part far from the reading surface on which it is set, such as a book, is scanned, a loose image corresponding to the part of the document far from the reading surface is obtained. In addition, as a thin light beam is not used, this type of the optical scanner is not suitable for accurately reading the document.
In the third example of the optical scanner unit, as a thin laser beam scans the document, the focal depth is large. In addition, as the laser beam can be focused so as to have a small spot size, fine images can be accurately read. However, a mechanism for moving the document scanned by the laser beam is needed, so that the size of the optical scanner unit is enlarged.
In addition, in the conventional optical scanner unit, a wavelength of the light emitted from the light source is constant. In a case where an image formed on the document has a color corresponding to approximately the same wavelength as the light, the image can hardly be read.